The present invention is directed to new and useful improvements in pressure filters and more particularly to a pressure filter used for separating a liquid from a solids slurry solution and concentrating the solids as a thickened underflow discharge from the filter.
In pressure filtration, a feed slurry is pumped into a pressure vessel and solids which are separated from the liquid collect as a filter cake on a filter medium while the filtrate passes through the filter medium into a filtrate compartment in the vessel. By selecting a filter medium of sufficiently fine pores the solids content of the filtrate can be reduced to a very low level. Periodically the solids are removed from the filter medium by the action of a backflow of filtrate from the filtrate compartment and allowed to settle at the bottom of the vessel from which it is removed through a sump discharge.
Pressure filters of the type mentioned are used for example in kraft pulp mill recausticizing systems for white liquor clarification and lime mud washing and thickening. In these filtration devices a slurry is fed under pressure into a cylindrical filter tank and the liquor is forced through suspended tubular filter elements which comprise perforated tubes having filter socks covering the outer surface thereof and to which surface adheres the lime mud filter cake. The clarified liquor enters a filtrate chamber located in the upper portion of the tank above the filter elements and is discharged therefrom to an overflow launder or directed to a head box located above the tank and from which head box the liquor flows to an appropriate storage tank.
In certain of these filters a pressurized air cushion is established in the filtrate chamber beneath a dome cover to assist in the cake discharge operation which is initiated by operation of a sump outlet valve to cause a rapid fall of pressure in the lower part of the filter tank. As the air cushion in the filtrate chamber expands the filtrate is forced back through the filter elements at a high flow rate to remove the cake therefrom for discharge through a sump outlet. In another similar type filter the backwashing of the filter cake from the filter elements is effected by dissipating the pressure in the filter tank and venting the filtrate chamber to the atmosphere causing the liquor to flow back through the filter elements by hydrostatic pressure to remove the filter cake from the surface of the elements. In these known filters the operation of the filter is automatic and cyclical usually lasting 4-6 minutes processing 310 to 925 gallons per minute depending upon the capacity of the filter tank.
Typical of these prior art filtration devices are disclosed in in U.S. Pat. Nos. 1,452,151; 2,035,592; 2,391,534; 3,356,215 and 4,243,533. In these patents pressure filters are disclosed, for example, for use in the filtering and clarification of white liquor in a kraft pulp mill process and for discharging a thickened lime mud solids discharge. In certain of these patents the liquor is pumped from the filter elements into a filtrate compartment which is comprised of a plurality of segmented filtrate compartments and wherein the backwashing is controlled by the operation of a rotary valve for periodically and sequentially backwashing a predetermined number of filter elements in accordance with its position relative to a filtrate compartment. In other of these patents the filter elements are backwashed by filtrate from the filtrate compartments or chambers by hydrostatic pressure upon dissipation of the pressure within the filtrate tank.
It is an object of the present invention to provide a novel pressure filter wherein the rate of filtration is increased while filtrate liquor clarity levels are maintained.
Another object is to provide a novel pressure filter wherein novel cake flushing and displacement means are utilized for assisting in cake removal from the filter cake for discharge from the filter tank.
Another object is to provide a novel method for operating a filter with high differential pressure across the filter elements to increase productivity and lower overall equipment costs.
A still further object is to provide a novel pressure filter and method of operation to effect high rate performance and requiring shorter residence time of solids in the solids underflow settling region and for providing forward flush means for removing the filter cake during cyclical operation of the filter.